1. Technological Field
The present disclosure relates generally to the field of data networking and telecommunications. More particularly, in one exemplary aspect, the present disclosure is directed to the intelligent scrambling of control symbols that are transmitted over a network in order to improve undesired electromagnetic emissions.
2. Description of Related Technology
HDMI (High-Definition Multimedia Interface) (see, inter alia, www.hdmi.org) is an exemplary dominant digital audio/video interface technology specified by the HDMI Founders. Current incarnations of the standard specify support for simple networking of digital audio/visual (A/V) interconnects, intended to be used primarily between an arbitrary assembly of multimedia “sources” (e.g., set-top boxes, DVD players, Blu-ray Disc players, video game consoles, computers or CPUs) and “sinks” (e.g., display monitors, home-theater system, etc.). This interconnection is generally unidirectional in nature; i.e., from source to sink, in current implementations.
The current revision of HDMI (HDMI 1.4) utilizes TMDS (Transition Minimized Differential Signaling) to transmit video, audio and auxiliary data over three main HDMI data lanes via one of three modes. FIG. 1 illustrates these TMDS modes in an exemplary 720×480p video frame. The three modes include: (1) a video data period in which the pixels of an active video line are transmitted; (2) a data island period in which audio and auxiliary data are transmitted; and (3) a control period which occurs between video and data island periods. HDMI utilizes TMDS in order to send 10-bit characters that are transmission minimized encoded.
During control periods, control symbols are transmitted repeatedly on the three HDMI main data lanes. HDMI defines four control symbols, representing the four values 0b00, 0b01, 0b10 and 0b11. The value transmitted on lane zero represents whether HSYNC, VSYNC, both or neither is/are being transmitted, and the values transmitted on lanes 1 and 2 are set to non-zero during preambles. Preambles are constructed from eight identical control symbols and are used to signify imminent transitions to data island periods or video data periods. Table 1 illustrates the preambles used for each data period type (i.e., TMDS mode).
TABLE 1CTL0CTL1CTL2CTL3Data Period Type1000Video Data Period1010Data Island PeriodAdditionally, Table 2 illustrates the control signal-assignment on each of the three main TMDS channels.
TABLE 2TMDSChannel(Lane)D0D10HSYNCVSYNC1CTL0CTL12CTL2CTL3The two control signals used for each of the TMDS channels are encoded into ten-bit codes as follows:
case (D1, D0):                0,0: q_out [9:0]=0b1.101010100;        0,1: q_out [9:0]=0b0010101011;        1,0: q_out [9:0]=0b0101010100;        1,1: q_out [9:0]=0b1010101011;        
endcase;
As can be seen from above, these ten-bit codes used for the four control symbols have predominant clock pattern content (i.e., a significant amount of transmissions of 0101 bit-pattern sequences). Furthermore, the control symbols for control values 0b10 and 0b11 are not DC balanced, which results in a significant baseline “wander” during the time that VSYNC is transmitted. Note also that VSYNC is often used in negative parity, which means that the VSYNC value transmitted is “0” when VSYNC is asserted, and “1” when VSYNC is not asserted (depending on the video frame specific resolution details).
HDMI also uses the unique high-transition content of these control symbols to distinguish them from other types of symbols used in HDMI. This is then used by the receiver to perform symbol alignment when first acquiring the incoming signal stream (so-called symbol alignment synchronization).
A result of the repeating of these control symbols during these control periods is the generation of significant electromagnetic emissions. Consequently, these HDMI implementations can become the source of electromagnetic interference (EMI) with nearby wireless services (e.g. Wi-Fi, PAN (e.g., Bluetooth) and cellular services). The resultant radio frequency interference can negatively impact, for example, an end user's Wi-Fi bandwidth and reduce cellular bandwidth or drop cellphone calls when the appropriate wireless services are used simultaneously with HDMI. Such a problem is exacerbated in devices in which HDMI is used in close proximity with other wireless services, such as in a laptop computer or a smart phone.
Accordingly, improved apparatus and methods are needed in order to address these EMI concerns with regards to, inter alia, control symbols such as those used in an exemplary HDMI implementation. Such improved apparatus and methods would ideally reduce the effects of EMI in situations such as when control symbols are transmitted in proximity to other wireless services. More generally, such apparatus and methods would provide for mechanisms that improve the characteristics of the symbols transmitted over the transmission medium, while advantageously leveraging existing hardware so as to minimize circuitry costs in future implementations.